LAN headroom: Designing for expansion
Robert McIntire
Published: 15 Apr 2002 14:30 BST
I've been involved in networking projects where the pace moved so quickly that one very important aspect of implementing a LAN was shelved for later or ignored altogether: the planning phase. There isn't always time to consider the big picture, but a sound planning and design phase can position your network implementation for growth and flexibility.
If the initial network implementation is small enough, it may be difficult to resist the urge to just toss in a switch and be done with it. But then what happens when it's time to grow? You'll most likely be forced to add other subnets and a router to handle internal and external routing to the Internet and other potential network partners. To help you nip network expansion woes in the bud, I'm going to provide a blueprint for designing LANs with plenty of headroom for future expansion.
Thinking ahead
As your small network quickly grows into a much larger entity, will it become a multitentacled monster or will you be able to accommodate growth in an orderly manner? The answer lies in addressing the need for expansion up front. By applying a little sound planning in this area, you can position the new network implementation to withstand even rapid growth.
As you develop your plan, you'll need to address a number of considerations, such as the number of workstations you must initially support and how many you will support after expansion. Be sure to take location into account--it can seriously affect the necessary cabling. For instance, if your network clients are located on multiple floors, how will you handle connectivity between floors? In this area, it's almost a given to follow standard design practices. This suggests a flexible cable plant, where connections on each floor are aggregated to a punch-down block in the distribution frame for that respective floor. Then, you mount switches at the distribution frame for each floor and run interfloor cabling to connect these directly to the core switch. When doing so, recommend fiber connections between switches to accommodate future traffic generated by many 100-MB workstation connections.
That's enough about cabling--but it does illustrate the complexity of some of the issues you'll need to tackle. For the purposes of this article, we'll consider a simple scenario with all PCs located on the same floor within maximum cable-length distance.
The scenario
Let's say you're starting off with a small network configuration that has a single Cisco switch and router. The switch has a fixed number of ports, but the router has basic capacity for expansion. You chose basic models for your initial small-scale LAN, for cost reasons. If your client or employer doesn't budget for extra infrastructure before implementation, that's okay. Look at it from a pay-as-you-grow approach. This detail, in itself, would normally mean little or no expansion room, but you can set it up to overcome the aspect of a fixed form-factor. Integrate VLANs into your design to prepare for the inevitable. Use such features as VLAN Trunking Protocol (VTP) and trunking. This will allow for easy expansion because when you add the next switch, setup will be minimal; it will inherit VLAN information from the original switch when you make it a client member of the VTP domain.
VLAN information
For more information on VLANs, take a look at Robert McIntire's article "VLANs and switching technology: A nuts-and-bolts approach to Cisco VLAN design."
First, assume that the basic setup has already been performed on both a Cat 3500 series switch and a 2600 series router. Then, look at the connections between devices. Naturally, the router will connect to both the Internet and the local LAN(s). The Internet connection is provided through an external To Switch Unit (TSU) connected to the serial interface in the router. Normally, at this point, I'd say something about security (perimeter networks, DMZ, firewalls, etc.), but that's a bit outside the scope for setting up a basic network with expansion in mind.
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